Palygorskite- and Halloysite-TiO2 nanocomposites: Synthesis and photocatalytic activity

Palygorskite-TiO2 nanocomposites were prepared by deposition of the anatase form of TiO2 on the clay surfaces using a sol-gel method with titanium isopropoxide as a precursor under hydrothermal treatment at 180 °C. The same procedure was followed in the formation of halloysite-TiO2 nanocomposites using a halloysite sample. Phase composition, particle morphology and physical properties of these samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and microanalysis by energy dispersive spectrometry (EDS), transmission electron microscopy (TEM), attenuated total reflection using Fourier transform infrared spectroscopy (ATR-FTIR) and N2 surface area analysis by BET. The photocatalytic activities of clay-titania nanocomposites in decomposing NOx gas were measured. After treating with TiO2, the halloysite and palygorskite samples showed mesopores of about 5.6 and 6.5 nm, respectively, while the macropores of halloysite (central hole in halloysite tubes) disappeared. The latter is attributed to the covering of the central hole in halloysite tubes by TiO2 nanoparticles and for that reason the pore size of the TiO2-treated halloysite was significantly smaller. The clay-titania samples showed significantly higher activity in decomposing NOx gas under visible-light irradiation (λ > 510 nm) and UV light irradiation (λ > 290 nm) compared to that of the standard commercial titania, P25.